In the chemical process industries, closed pressure loadable chambers are used to process reactants where elevated temperatures and pressures are required to carry out varied unit processes and operations. One type of "pressure cooker" commonly employed on an industrial scale is an autoclave. It has long been used medically in the sterilization of drugs, vaccines, instruments, and for other products which are subject to modification or purification through heat and pressure.
Autoclaves commonly have lidded removable (or hingeable) tops which engage a peripheral sealing gasket which rides on a usually circular lip of the open vessel. The lid is secured to the pressure chamber upper edge via a plurality of headless bolts sized to threadingly engage a plurality of tapped holes in the lid member on its so-called hold-down ring. As the headless bolts are tightened from the outside of the vessel, they force the lid to exert a compressive force on the underlying circular gasket until the desired degree of hermetic seal is achieved. In actual operation of a sealed autoclave lid, the practical hazard of "frozen" bolts is not uncommon; nor is it one that is readily avoided, especially where processed materials create reactive vapors that attack the locking bolts from the exposed underside.
Aggravating conditions are elevated temperatures and pressures, extended corrosive operations and inadequate lubrication, any one of which can produce a "frozen" bolt. "Stuck" bolts often occur due to galling caused by thread imperfections, or occluded foreign materials, or leakage of vessel contents containing particulate materials conducive to producing high frictional loads upon separation.
In many instances, the successful removal of a "stuck" bolt by excessive wrenching efforts results in thread damage to both mating elements of the standard closure assembly, that is, the threaded bolt and the tapped lid passages receiving the bolt. Indeed, if the bolts cannot be loosened, or their wrenching surfaces become too damaged, the bolts must be machined out of the threaded tap while in the lid ring.
While locking bolts are comparatively economical and thus expendable, the involved closure ring of the lid is not, since it is fabricated in compliance with the boiler and pressure vessel codes. If these are compromised by destructive bolt removal steps, replacement of a closure ring is very costly. This takes extended time to remedy since the replacements will require hydrotesting, inspection and certification to comply with the codes.
Among the prior art approaches to closure means is a screw mounting means for use with an electrical outlet box. A clip member is to be slidably engaged in a slotted aperture adjacent an unthreaded screw receiving bore hole (see U.S. Pat. No. 3,955,463 to Hoehn). A threaded screw is fastened in the bore hole by being manually thrust into the bore hole without turning and engaging a spring detent with the screw threads.
Another prior art closure means is the so-called standoff assembly of U.S. Pat. No. 4,117,261 to Blevins, et al., in which a socket, sized to accept an insert, is fastened through a hole in a structural member. A shoulder is provided on the insert. The shoulder is located fully contiguous to the lip of the socket when the insert is fully bottomed. This form of plate locking device is not adaptable to an autoclave lid cover involving pressure services because severe conditions of elevated temperature (circa 750.degree. F.) and attendant pressures (circa 4000 psig) cannot be accommodated by the referenced device.
Still another means for a structural closure is the removable fastening assembly of U.S. Pat. No. 4,606,688 to Moran, et al., which assembly is able to secure an externally threaded member to an article to an opening therein. Though this teaching is based on adjustable position take up screws, the assembly appears to be too fragile to be useful with heavy lidded pressure vessels.
It is a principal object of the invention to provide an economical and reliable closure assembly for pressure loaded vessels which is the functional equivalent of known peripheral locking bolt and tapped hole combination.
It is another object of the invention to provide an inexpensive and expendable closure assembly adaptable to existing autoclave lidded members which does not compromise the integrity of the compressive load carrying member and also includes readily replaceable elements in the event of a failure of the closure assembly release.
It is still another object to substitute the plural threaded tap holes being fabricated in the lid member (hold-down ring) with simple cylindrical bore holes (sockets) on the lid underside that will accommodate the novel closure assemblies of the present invention in the manner of the so-called "captive nut" fasteners, which are normally used where access to the "nut" side of the locking member is unavailable.